1. Field of the Invention
This invention concerns a tank for storing liquids and, more particularly, a flexible double-walled tank that is easily transportable when empty.
2. Discussion of the Background
Flexible tanks for liquids are generally known in the art and are principally used for mobile and temporary emergency purposes. Such tanks are therefore better suited for transportation than tanks with a defined, i.e., fixed shape. Tanks with a flexible outer tubular polypropylene cloth present a quasi-defined shape when filled. This shape is determined by the cut of the tubular polypropylene cloth and the kind of holding fixture used for the tank. It is observed that the material tensions in the tubular polypropylene cloth must remain within an admissible range under all operating conditions, since an excess material tension can lead to the tearing of the tubular polypropylene cloth. The areas of edges and corners of a tank are subject to material tensions, and these areas must be especially reinforced in the construction and manufacture of the tubular polypropylene cloth. The greater the tank's volume, the greater the forces attacking the tubular polypropylene cloth. Consequently, the requirements for stress capacity of the tank walls grow as well. The stress capacity of the tank walls can be raised by using a thicker material. However, this reduces the flexibility of the tank and some related advantages, such as easy transportability. Furthermore, processing of thicker wall materials cannot be effected with the same technologies as are suitable for the processing of thin flexible materials.
Tanks for liquids are generally hung in fixtures. For example, cylindrical tanks often are mounted to hang vertically in supporting devices. Such holding fixtures must be capable of supporting the entire weight of the full tank and have to be dimensioned correspondingly. Accordingly, the holding fixtures are bigger, more unwieldy, and heavier than desired and, in fact, are contradictory to the demand for a tank of low weight and easy transportability.
Consequently, there have been limits to the size of flexible tanks for liquids, i.e., the bigger the tank volume, the thicker and, therefore, stiffer must the tank walls be constructed and the heavier and bulkier are the holding fixtures. Thus, the advantageous features such as low weight and easy folding when empty and, consequently, good transportability, were lost.